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1.
Here, we report on a kinetically controlled vapour phase condensation experiment using a low-calcium Ca–Fe–SiO–H2–O2 vapour. Under these conditions of extreme disequilibrium, the condensate properties become predictable. They are amorphous solids with (predictable) deep metastable eutectic compositions. This study also shows how chemical evolution of the condensate grains will lead to chemically complex amorphous solids. The highly disordered structure of the deep metastable eutectic condensates is the very key to this predictable chemical evolution to grains with a silicate mineral composition, yet being amorphous. We compare our results with astronomical observations of dust around young stellar objects.  相似文献   

2.
O. Gomis  G. Strazzulla 《Icarus》2005,177(2):570-576
In this work we report on new experiments of ion irradiation of water ice deposited on top of solid carbonaceous materials to study the production of CO2 at the interface ice/refractory material and discuss the possibility that this mechanism accounts for the quantity of CO2 ice detected on the surfaces of the Galilean satellites. The used experimental technique has been in situ infrared spectroscopy. We have irradiated thin films of H2O frost on carbonaceous layers with 200 keV of He+ and Ar+, and 30 keV of He+ at 16 and 80 K. The used carbonaceous layers have been asphaltite, a natural bitumen, and solid organic residues obtained by irradiation of frozen benzene. In both cases the results show that CO2 is produced very efficiently after irradiation obtaining a maximum quantity of the order of . These results are, also quantitatively similar, to those recently obtained for water ice deposited on amorphous carbon films [Mennella, V., Palumbo, M.E., Baratta, G.A., 2004. Formation of CO and CO2 molecules by ion irradiation of water ice covered hydrogenated carbon grains. Astrophys. J. 615, 1073-1080]. Thus we suggest that, whatever is the carbonaceous residue, CO2 will be produced efficiently by the studied process. These results have interest in the context of the surfaces of the icy Galilean satellites in which CO2 has been detected mainly trapped in the non-ice material, not in the pure water ice. We suggest that radiolysis of mixtures of water ice and refractory carbonaceous materials is the primary formation mechanism responsible for the CO2 formation on the surfaces of the Galilean satellites.  相似文献   

3.
The formation of CO2 ice clouds in the upper atmosphere of Mars has been suggested in the past on the basis of a few temperature profiles exhibiting portions colder than CO2 frost point. However, the corresponding clouds were never observed. In this paper, we discuss the detection of the highest clouds ever observed on Mars by the SPICAM ultraviolet spectrometer on board Mars Express spacecraft. Analyzing stellar occultations, we detected several mesospheric detached layers at about 100 km in the southern winter subtropical latitudes, and found that clouds formed where simultaneous temperature measurements indicated that CO2 was highly supersaturated and probably condensing. Further analysis of the spectra reveals a cloud opacity in the subvisible range and ice crystals smaller than 100 nm in radius. These layers are therefore similar in nature as the noctilucent clouds which appear on Earth in the polar mesosphere. We interpret these phenomena as CO2 ice clouds forming inside supersaturated pockets of air created by upward propagating thermal waves. This detection of clouds in such an ultrararefied and supercold atmosphere raises important questions about the martian middle-atmosphere dynamics and microphysics. In particular, the presence of condensates at such high altitudes begs the question of the origin of the condensation nuclei.  相似文献   

4.
The widespread deposition of CO2 ice on the martian polar caps in winter is readily visible from Earth and has been extensively studied from orbit. As the surface cools during polar night, CO2 condenses directly out of the atmosphere at a rate that establishes equilibrium between radiative loss and latent heat of condensation. Since radiative loss is strongly geometry-dependent, the CO2 frost will grow most rapidly on exposed surfaces and more slowly in depressions. Positive feedback will cause a dramatic enhancement of the relief of the underlying topography and a corresponding reduction in the average bulk density. The resulting surface will be highly textured and riddled with perforations.  相似文献   

5.
The mid-infrared spectra of mixed vapor deposited ices of CO2 and H2O were studied as a function of both deposition temperature and warming from 15 to 100 K. The spectra of ices deposited at 15 K show marked changes on warming beginning at 60 K. These changes are consistent with CO2 segregating within the ice matrix into pure CO2 domains. Ices deposited at 60 and 70 K show a greater degree of segregation, as high as 90% for 1:4 CO2:H2O ice mixtures deposited at 70 K. As the ice is warmed above 80 K, preferential sublimation of the segregated CO2 is observed. The kinetics of the segregation process is also examined. The segregation of the CO2 as the ice is warmed corresponds to temperatures at which the structure of the water ice matrix changes from the high density amorphous phase to the low density amorphous phase. We show how these microstructural changes in the ice have a profound effect on the photochemistry induced by ultraviolet irradiation. These experimental results provide a framework in which observations of CO2 on the icy bodies of the outer Solar System can be considered.  相似文献   

6.
In order to study the stability of martian climate, we constructed a two-dimensional (horizontal-vertical) energy balance model. The long-term CO2 mass exchange process between the atmosphere and CO2 ice caps is investigated with particular attention to the effect of planetary ice distribution on the climate stability. Our model calculation suggests that high atmospheric pressure presumed for past Mars would be unstabilized if H2O ice widely prevailed. As a result, a cold climate state might have been achieved by the condensation of atmospheric CO2 onto ice caps. On the other hand, the low atmospheric pressure, which is buffered by the CO2 ice cap and likely close to the present pressure, would be unstabilized if the CO2 ice albedo decreased. This may have led the climate into a warm state with high atmospheric pressure owing to complete evaporation of CO2 ice cap. Through the albedo feedback mechanisms of H2O and CO2 ices in the atmosphere-ice cap system, Mars may have experienced warm and cold climates episodically in its history.  相似文献   

7.
Models are developed to describe the photochemistry of ozone on Mars. Catalytic reactions involving H, OH and HO2 play a major role at low latitudes where they ensure a vertical column density for O3 of less than 2 × 10?4 cm atm. The source for odd hydrogen (H + OH + HO2) is relatively smaller at high latitudes in winter due to the small concentrations of H2O present there at that time. Odd hydrogen is also efficiently removed from the high-latitude winter atmosphere by condensation of H2O2. The role of catalytic chemistry is reduced accordingly and the vertical column density of O3 may be as large as 5.7 × 10?3 cm atm in accord with earlier observations carried out by Barth and co-workers with instruments on Mariner 9.  相似文献   

8.
It has been suggested that inclusions of CO2 or CO2 clathrate hydrates may comprise a portion of the polar deposits on Mars. Here we present results from an experimental study in which CO2 molecules were trapped in water ice deposited from CO2/H2O atmospheres at temperatures relevant for the polar regions of Mars. Fourier-Transform Infrared spectroscopy was used to monitor the phase of the condensed ice, and temperature programmed desorption was used to quantify the ratio of species in the generated ice films. Our results show that when H2O ice is deposited at 140-165 K, CO2 is trapped in large quantities, greater than expected based on lower temperature studies in amorphous ice. The trapping occurs at pressures well below the condensation point for pure CO2 ice, and therefore this mechanism may allow for CO2 deposition at the poles during warmer periods. The amount of trapped CO2 varied from 3% to 16% by mass at 160 K, depending on the substrate studied. Substrates studied were a tetrahydrofuran (C4H8O) base clathrate and Fe-montmorillonite clay, an analog for Mars soil. Experimental evidence indicates that the ice structures are likely CO2 clathrate hydrates. These results have implications for the CO2 content, overall composition, and density of the polar deposits on Mars.  相似文献   

9.
The mostly carbon dioxide (CO2) atmosphere of Mars condenses and sublimes in the polar regions, giving rise to the familiar waxing and waning of its polar caps. The signature of this seasonal CO2 cycle has been detected in surface pressure measurements from the Viking and Pathfinder landers. The amount of CO2 that condenses during fall and winter is controlled by the net polar energy loss, which is dominated by emitted infrared radiation from the cap itself. However, models of the CO2 cycle match the surface pressure data only if the emitted radiation is artificially suppressed suggesting that they are missing a heat source. Here we show that the missing heat source is the conducted energy coming from soil that contains water ice very close to the surface. The presence of ice significantly increases the thermal conductivity of the ground such that more of the solar energy absorbed at the surface during summer is conducted downward into the ground where it is stored and released back to the surface during fall and winter thereby retarding the CO2 condensation rate. The reduction in the condensation rate is very sensitive to the depth of the soil/ice interface, which our models suggest is about 8 cm in the Northern Hemisphere and 11 cm in the Southern Hemisphere. This is consistent with the detection of significant amounts of polar ground ice by the Mars Odyssey Gamma Ray Spectrometer and provides an independent means for assessing how close to the surface the ice must be. Our results also provide an accurate determination of the global annual mean size of the atmosphere and cap CO2 reservoirs, which are, respectively, 6.1 and 0.9 hPa. They also indicate that general circulation models will need to account for the effect of ground ice in their simulations of the seasonal CO2 cycle.  相似文献   

10.
The interaction of carbon dioxide and amorphous water ice at 95 K is studied using transmission infrared spectroscopy. Samples are prepared in two ways: co-deposition of the gases admitted simultaneously or sequential deposition, in which amorphous water ice (ASW) is grown first and CO2 vapor is added subsequently. In either case, a fraction of the CO2 molecules is found to interact with water in a way that gives rise to shifts and splittings in the infrared bands with respect to those of a pure CO2 solid. In co-deposition experiments, a larger amount of carbon dioxide is trapped within the amorphous water than in sequential deposition samples, where a substantial proportion of molecules appears to be trapped in macropores of the ASW. The specific surface area of sequential samples is evaluated and compared to previous literature results. When the sequential samples are heated to 140 K, beyond the onset temperature at which water ice undergoes a phase transition, the CO2 molecules at the pores relocate inside the bulk in a structure similar to that found in co-deposited samples, as deduced by changes in the shape of the CO2 infrared bands.  相似文献   

11.
The reactivity of C2(X1Σ+g) with simple saturated (CH4, C2H6 and C3H8) and unsaturated (C2H2 and C2H4) hydrocarbons has been studied in the gas phase over the temperature range 24-300 K using the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme or Reaction Kinetics in a Uniform Supersonic Flow) technique. All reactions have been found to be very rapid in this temperature range and the rate coefficients are typically ?10−10 cm3 molecule−1 s−1 with the exception of methane for which the rate coefficient is one order of magnitude lower: ∼10−11 cm3 molecule−1 s−1. These results have been analyzed in terms of potential destruction sources of C2(X1Σ+g) in the atmospheres of Titan and the Giant Planets. It appears that the rate coefficient of the reaction 1C2 + CH4 should be updated with our new data and that reactions with C2H2, C2H4 and C2H6 should also be included in the existing photochemical models.  相似文献   

12.
We report observation and analysis of a high-resolution 2.87-3.54 μm spectrum of the southern temperate region of Saturn obtained with NIRSPEC at Keck II. The spectrum reveals absorption and emission lines of five molecular species as well as spectral features of haze particles. The ν2+ν3 band of CH3D is detected in absorption between 2.87 and 2.92 μm; and we derived from it a mixing ratio approximately consistent with the Infrared Space Observatory result. The ν3 band of C2H2 also is detected in absorption between 2.95 and 3.05 μm; analysis indicates a sudden drop in the C2H2 mixing ratio at 15 mbar (130 km above the 1 bar level), probably due to condensation in the low stratosphere. The presence of the ν3+ν9+ν11 band of C2H6 near 3.07 μm, first reported by Bjoraker et al. [Bjoraker, G.L., Larson, H.P., Fink, U., 1981. Astrophys. J. 248, 856-862], is confirmed, and a C2H6 condensation altitude of 10 mbar (140 km) in the low stratosphere is determined. We assign weak emission lines within the 3.3 μm band of CH4 to the ν7 band of C2H6, and derive a mixing ratio of 9±4×10−6 for this species. Most of the C2H6 3.3 μm line emission arises in the altitude range 460-620 km (at ∼μbar pressure levels), much higher than the 160-370 km range where the 12 μm thermal molecular line emission of this species arises. At 2.87-2.90 μm the major absorber is tropospheric PH3. The cloud level determined here and at 3.22-3.54 is 390-460 mbar (∼30 km), somewhat higher than found by Kim and Geballe [Kim, S.J., Geballe, T.R., 2005. Icarus 179, 449-458] from analysis of a low resolution spectrum. A broad absorption feature at 2.96 μm, which might be due to NH3 ice particles in saturnian clouds, is also present. The effect of a haze layer at about 125 km (∼12 mbar level) on the 3.20-3.54 μm spectrum, which was not apparent in the low resolution spectrum, is clearly evident in the high resolution data, and the spectral properties of the haze particles suggest that they are composed of hydrocarbons.  相似文献   

13.
The Mars Orbiter Camera onboard the Mars Global Surveyor has obtained several images of polygonal features in the southern polar region. In images taken during the end of the southern spring, when the surrounding surface is free of the seasonal frost, CO2 ice still appears to be present within the polygonal troughs. In Earth's polar regions, polygons such as these are indicative of water ice in the ground below. We analyzed the seasonal evolution of the thermal state and the CO2 content of these features. Our 2-D model includes condensation and sublimation of the CO2 ice, a self consistent treatment of the variations of the thermal properties of the regolith, and the seasonal variations of the local atmospheric pressure which we take from the results of a general circulation model. We find that the residence time of seasonal CO2 ice in troughs depends not only on atmospheric opacity and albedo of the CO2 ice, but also and most significantly on the distribution of water ice in the regolith. Optical properties of the atmosphere and surface CO2 ice can be independently obtained from observations. To date this is not true about the distribution of water ice below the surface. Our analysis quantifies the dependence of the seasonal cycle of the CO2 ice within the troughs on the assumed distribution of the water ice below the surface. We show that presence of water ice in the ground at a depth smaller than the depth of the troughs reduces winter condensation rate of CO2 ice. This is due to higher heat flux conducted from the water ice rich regolith toward the facets of the troughs.  相似文献   

14.
The variation in elemental depletions seen in diffuse clouds in the interstellar medium is shown to be consistent with the exposure of grains to weak shocks. Shocks remove a weakly bound layer of amorphous oxide material from refractory cores. Depletions are shown to be the result of the incorporation of elements in refractory cores formed during equilibrium condensation in high temperature sources together with accretion under low (T1000K) temperature conditions. Evidence is presented for the high temperature condensation of CaAlTi grains. In addition to refractory CaAlTi particels, a refractory condensate containing 20% of available Mg, 60% of Si and 90% of Fe is inferred from data on depletions. An amorphous layer with a composition (2MgO) (SiO) condenses on these cores.  相似文献   

15.
Mars General Circulation Model (GCM) simulations are presented to illustrate the importance of the ice emissivity of the seasonal CO2 polar caps in regulating the effects of airborne dust on the martian CO2 cycle. Simulated results show that atmospheric dust suppresses CO2 condensation when the CO2 ice emissivity is high but enhances it when the CO2 ice emissivity is low. This raises the possibility that the reason for the repeatable nature of the CO2 cycle in the presence of a highly variable dust cycle is that the CO2 ice emissivity is “neutral” - the value that leads to no change in CO2 condensation with changing atmospheric dust. For this GCM, the “neutral” emissivity is approximately 0.55, which is low compared to observed cap emissivities. This inconsistency poses a problem for this hypothesis. However, it is clear that the CO2 ice emissivity is a critical physical parameter in determining how atmospheric dust affects the CO2 cycle on Mars.  相似文献   

16.
We present near-IR spectra of solid CO2 in H2O and CH3OH, and find they are significantly different from that of pure solid CO2. Peaks not present in either pure H2O or pure CO2 spectra become evident when the two are mixed. First, the putative theoretically forbidden CO2 (2ν3) overtone near 2.134 μm (4685 cm−1), that is absent from our spectrum of pure solid CO2, is prominent in the spectra of H2O/CO2=5 and 25 mixtures. Second, a 2.74-μm (3650 cm−1) dangling OH feature of H2O (and a potentially related peak at 1.89 μm) appear in the spectra of CO2-H2O ice mixtures, but are probably not diagnostic of the presence of CO2. Other CO2 peaks display shifts in position and increased width because of intermolecular interactions with H2O. Warming causes some peak positions and profiles in the spectrum of a H2O/CO2=5 mixture to take on the appearance of pure CO2. Absolute strengths for absorptions of CO2 in solid H2O are estimated. Similar results are observed for CO2 in solid CH3OH. Since the CO2 (2ν3) overtone near 2.134 μm (4685 cm−1) is not present in pure CO2 but prominent in mixtures, it may be a good observational (spectral) indicator of whether solid CO2 is a pure material or intimately mixed with other molecules. These observations may be applicable to Mars polar caps as well as outer Solar System bodies.  相似文献   

17.
Experimental results on the spectral identification of new infrared absorption features and the changes of their absorbances produced through vacuum ultraviolet-extreme ultraviolet (VUV-EUV) photon-induced chemical reactions in the C2H2-H2O mixed ices at 10 K are obtained. To the best of our knowledge, this is the first time that EUV photons have been employed in the study of the photolysis of ice analogues. Two different compositions, i.e., C2H2:H2O=1:4 and 1:1, were investigated in this work. A tunable intense synchrotron radiation light source available at the Synchrotron Radiation Research Center, Hsinchu, Taiwan, was employed to provide the required VUV-EUV photons. In this study, the photon wavelengths selected to irradiate the icy samples corresponded to the prominent solar hydrogen, helium, and helium ion lines at 121.6 nm, 58.4 nm, and 30.4 nm, respectively. The photon dosages used were typically in the range of 1×1015 to 2×1017 photons. Molecular species produced and identified in the ice samples at 10 K resulting from VUV-EUV photon irradiation are mainly CO, CO2, CH4, C2H6, CH3OH, and H2CO. In addition to several unidentified features, we have tentatively assigned several absorption features to HCO, C3H8, and C2H5OH. While new molecular species were formed, the original reactants, i.e., H2O and C2H2, were detectably depleted due to their conversion to other species. The new chemical species produced by irradiation of photons at 30.4 nm and 58.4 nm can be different from those produced by the 121.6-nm photolysis. In general, the product column density of CO reaches saturation at a lower photon dosage than that of CO2. Furthermore, the production yield of CO is higher than that of CO2 in the photon irradiation. In the present study, we also observe that the photon-induced chemical reaction yields are high using photons at 30.4 and 58.4 nm. The results presented in this work are essential to our understanding of chemical synthesis in ice analogues, e.g., the cometary-type ices and icy satellites of planetary systems.  相似文献   

18.
We have investigated the role of several ion-molecule reactions in the conversion of N2O5 to HNO3. In the proposed conversion, an N2O5 molecule would react with an H2O molecule clustered to an inert ion to produce two HNO3 molecules. Subsequent clustering of an H2O molecule to the inert ion would make the reaction catalytic. If such an ion-catalysed conversion of N2O5 to HNO3 occurs, it would probably play a role in the stratospheric chemistry at high latitudes in winter. In this paper we present reaction rate constant measurements made in a flowing afterglow apparatus for hydrated H3O+, H+(CH3CN)m (m = 1, 2, 3), and several negative ions reacting with N2O5. Slow rate constants were found for these ions for hydration levels that are predominant in the stratosphere. With the known stratospheric ion density, these slow rate constants preclude significant N2O5 conversion by ion-molecule reactions.  相似文献   

19.
Within the framework of classical nucleation theory we have shown that Mg2SiO4 grains (radii of nearly 0.1 μm) which are likely candidates as condensation products in circumstellar shells around late-type stars must condense at about 750 K to remain amorphous. If the condensation temperature is about 1000 K the newly formed grains must be rather small to rest in the amorphous state during cooling. Submicron sized MgSiO3grains are entire stable against crystallization for Tcond ≤ 1000 K.  相似文献   

20.
The production of Fe2SiO4 (fayalite) crystalline grains was performed by two processes, namely, grain formation in a plasma field by evaporating a mixture powder of Fe and SiO and heat treatment of the product collected on the radio-frequency (RF) electrode side. Fe grains <20 nm in size covered with an amorphous SiO layer selectively formed Fe2SiO4 grains by heating at 800 °C. By heating at 600 °C, in addition to the formation of Fe2SiO4 crystal grains, the FeO phase appeared. The doping effect of excited oxygen in a plasma field into the Fe small grains may be the trigger on the formation of fayalite through the FeO phase formation. The present experimental result suggests that the probability of Fe2SiO4 grain formation in space is low.  相似文献   

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